1. Field of the Invention
The present invention relates to a full duplex communication system, and more particularly, to an echo cancellation device for use in a full duplex communication system.
2. Description of the Prior Art
As technology advances, network applications have become more and more popular. Network bandwidth requirements are also increasing as the transmission speed of data transmission standards such as Ethernet have raised from 10/100 Mps to above 1 Gbps. As is well known in the art, each port of a 1 Gbps fast Ethernet device has four channels, wherein each of the four channels has a transceiver.
Please refer to FIG. 1. FIG. 1 depicts a simplified schematic diagram of a conventional transceiver 100 in a channel of a 1 Gbps Ethernet device. In general, the transceiver 100 is coupled to a twisted pair 118 via a line interface 116. As shown in FIG. 1, the transceiver 100 comprises a transmitter section 104 and a receiver section 106. The transmitter section 104 has a digital-to-analog converter (DAC) 108 for converting a transmit signal (a near-end signal) into analog form. The analog transmit signal is then transmitted to a far-end network device via the line interface 116 and the twisted pair 118. The receiver section 106 has an analog-front-end (AFE) circuit 112 for processing a receive signal (a far-end signal) received from the line interface 116, and an analog-to-digital converter (ADC) 114 for converting the processed signals into digital form. The digital signal is then sent to following stages for further processing. The 1 Gbps Ethernet device and the far-end network device both simultaneously utilize four channels where each channel simultaneously performs transmitting and receiving operations. As a result, the 1 Gbps Ethernet device is a full duplex communication system.
As mentioned above, each channel of the 1 Gbps Ethernet device simultaneously performs transmitting and receiving operations. When the channel is transmitting, the signals received from the channel are affected by the transmission, and this phenomenon is known as echo impairment. In order to reduce echo impairment in a communication system, an echo cancellation device 110 and an echo cancellation resistor Rp are usually employed in the conventional transceiver 100. The echo cancellation device 110 is usually a DAC for generating a cancellation signal that corresponds to the transmit signal output from the DAC 108 in order to cancel the effects of the transmit signal on the receiver section 106 and thereby achieve echo cancellation.
Please refer to FIG. 2, which is an equivalent circuit diagram of the conventional transceiver 100 of FIG. 1. The electrical equivalence of the DAC 108 and the echo cancellation device 110 are current sources Id and Ic, respectively. In order to achieve echo cancellation for the receiver section 106, the effect caused by the output of the current source Id must be canceled by the output of the current source Ic.
Please refer to FIG. 3, which is a small signal model for the equivalent circuit diagram of FIG. 2. In FIG. 2, Zo is the equivalent output impedance of the transmitter section 104, and Zi is the equivalent input impedance of the receiver section 106. Vo is the output signal of the transceiver 100 and also the transmit signal output from the transmitter section 104. Vi is the echo on the receiver section 106 caused by the transmit signal. In the conventional art, the echo cancellation device 110 regards the equivalent output impedance Zo as a load resistor Re, and the resistance of the resistor Re consists of a matching resistor Rm, which is used to match impedance, and an equivalent resistor Rc of the channel coupled to the transceiver 100. From the small signal model shown in FIG. 3, the following formula can be obtained:
                              V          i                =                              -                          Zi              ⁡                              [                                  IdZo                  +                                                            (                                              Zo                        +                        Rp                                            )                                        ⁢                    Ic                                                  ]                                                          Rp            +            Zi            +            Zo                                              (        1        )            
In order to cancel the echo effect, Vi should be equal to 0, which satisfies:IdZo+(Zo+Rp)Ic=0   (2)
From formula (2), it is known that the relationship between Ic and Id is:
                    Ic        =                                            -              Zo                                      Rp              +              Zo                                ⁢          Id                                    (        3        )            
That is to say, if Ic and Id satisfy formula (3), the echo effect will be completely cancelled.
However, as mentioned above, the echo cancellation device 110 regards the equivalent output impedance Zo as the load resistor Re, which consists of the matching resistor Rm and the equivalent resistor Rc of the channel, but ignores an unavoidable parasitic capacitance effect that will occur in a practical implementation. Obviously, the echo effect of the transceiver 100 cannot be effectively reduced to the lowest level if the output impedance Zo is simply viewed as the load resistor Re in the prior art.
Furthermore, from FIG. 3, it is known that Vo>Vi. As the working voltage of transceiver ICs becomes lower and lower, both Vo and Vi also become lower and lower. If the working voltage of a transceiver IC becomes lower than a specific level, MOS transistors of the echo cancellation device 110 (electrically equivalent to the current source Ic) may become unable to maintain operation in the saturation region and therefore result in signal distortion in the conventional echo cancellation device 110.